The present invention relates to the techniques of graining an aluminum surface by using alternate current and suitable electrolytes for preparing printing matrixes.
As is known, in the field of the aluminum surface treatment, better known as graining, such technique consists of roughening the metal surface, so as to condition it, on the one hand, for retaining the photosensitive layer which will thus better adhere to the substrate during the printing stage, and on the other hand, for retaining water in the non-printing areas. The principle of the offset printing is based on the chemical-physical equilibrium between hydrophilic and hydrophobic parts and the basis of such equilibrium is established by a good technique of roughening the metal substrate surface.
By a "good roughening technique" is meant a technique capable of uniformly acting on the aluminum surface, so as to create a series of holes evenly spaced apart one from the other, having a frequency per unit surface as high as possible, and depths as homogeneous as possible. The reason of these requirements resides in the fact that the higher is the frequency of the holes produced by graining, the higher will be the adhesion of the photosensitive layer on the metal and thus the number of copies which the printing plate will be capable of producing. Moreover, for the same reason the higher will be the wettability with water of the nonprinting areas.
This will obviously contribute in the utmost manner to the separation of the lipophilic portion, i.e. printing portion, and hydrophilic non-printing portion. This because during the printing stage the ink will be fixed on the lipophilic areas which, as said, form the photosensitive layer.
Another very important parameter, at the same frequency of number of holes produced per unit surface, consists of the average depth of the same. In this respect it is to be stated that the manufacturers of printing plates try to reach a compromise between shallow holes which produce the so called fine grain, allow for a better reproduction of the image to be printed, require minor amounts of water, but which at the same time loose more easily the veil of absorbed water with the risk of forming the so called ink veil, and deep holes which produce the so called coarse grain and which, if on the one hand do not allow for the same good reproduction as the fine grain, allow for a greater wettability of the non-printing areas and, at the end, for a greater reliability for the printer.
The depth of the holes produced in the graining stage is measured with an apparatus called "roughness-meter" (profilemeter) having a very sensitive tip which is passed on the coarsened surface. The signal recorded by this tip is electronically amplified and gives a measure of the hole depth.
The most widely used value for the roughness measurement is Ra (average roughness) which indicates the average of the distances of the surface profile above and below a reference line defined as a line so drawn that the sum of the areas encompassed by the surface profile above the line is equal to the sum of those below the line.
Ra is normally measured in microns (micro-meters).
Now, as is known to those skilled in the art, the above disclosed coarsening effect, better known as graining, can be obtained with a simple aqueous solution of hydrochloric acid (HCl) with concentrations ranging from 3g/1 to 15g/1, by passing alternate current between the aluminum plate and a counter-electrode thereof at a current density of 2 to 5 Ampere/sq.dm for 30" to 120".
Actually, by varying the various cited parameters it is lo possible to obtain with this system a variation of the depth of the graining holes, but in order to obtain an acceptable homogeneity of the surface it is necessary to monitor very carefully the concentration of the acid and of the dissolved aluminum which is present in chloride form. Besides, at the lower concentration limits of HCl the obtained grain is fine, but there are ample areas completely and inhomogeneously flat. At high concentrations, the coarsening homogeneity is greater, but there are great probabilities of metal "dust" being formed which is then difficult to remove.
Mixed electrolytes, such as HCl and phosphoric acid, give better results, in that a lower Ra value and higher homogeneity can be obtained, but they raise rather big problems of formation of black aluminum metal powder, just called "dust" in the field.
Another electrolyte used with good results is nitric acid which however presents serious problems of environment polution in so far as treatment of gaseous and liquid effluents is concerned.
Certain patents, see GB 1598701, cite instead electrolytes still comprising hydrochloric acid and also comprising monocarboxylic acids with 1 to 4 carbon atoms.
According to that patent the grain thus obtained is more satisfactory, can be better controlled in view of obtaining increasing values on Ra scale, and is more homogeneous than that produced through previously known techniques, however the electrolytes involved smell disagreeably and are thus undesirable from a hygienic point of view.